1. Field of the Invention
The present invention is related to personal digital assistants (PDAs) and more particularly to a user input interface for a PDA or the like.
2. Background Description
Portable computing devices, such as what is normally referred to as a personal digital assistant (PDA), are increasing in popularity. A typical PDA is a limited function microcomputer provided with a pressure sensitive liquid crystal diode (LCD) display (a touch pad or a touch screen) for input and output (I/O). PDAs are being adapted for wireless Internet communication, e.g., using a modem for e-mail and web browsing. Further, for text input PDAs are known that have a specialized stroke based alphabet interface, e.g., Graffiti®, a selectable on-screen QWERTY keypad, or an expansion pack keyboard.
As these portable devices become smaller and more specialized, text input has become more difficult and less practical. Typical prior art handwriting recognition software may require users to learn special characters or effect a handwriting style in order to enter text. Text input using the Graffiti® unistroke (i.e., written with a single pen trace) alphabet can be un-natural because it requires users to adhere to strict rules that restrict character shapes; text input using an on-screen QWERTY keypad is somewhat clumsy because only small reductions in size can be made to keyboards before they become awkward to use. An expansion keyboard is impractical for on-the-go input. With either, the tapping on individual characters or the typing is less desirable than being able to handwrite notes or messages. Meanwhile, the demand for PDA information exchange, e-mail and internet access requires entry and retrieval of increasing amounts of data with the handheld device.
Natural handwriting recognition (HWR) programs have been developed to add to function and usefulness to PDAs and are crucial to the growth of mobile computing in the communications field. To use handwriting recognition software, such as Transcriber (formerly known as CalliGrapher) from Microsoft Corporation, the user writes a message anywhere on the PDA screen, i.e., on top of any displayed application and system elements. Alternatively, the user can write in a designated input area using handwriting recognition software such as QuickPrintPro™ from Motorola, Inc., Lexicus division.
In order to handle unconstrained handwritten input (i.e., written in cursive style, print style, or using a combination of both) adequately, typically HWR programs such as those mentioned above, employ recognition algorithms that rely on dictionaries to constrain the search decoding space—i.e., the space of possible letter sequences that can be matched to a given input ink. Users, however, often need to write otherwise non-sensical words (e.g. an e-mail address) that are outside of a typical system dictionary. For these situations the dictionary is useless and under some circumstances using a dictionary may actually impede correct recognition. However, HWR systems that are not, at least in part, dictionary based cannot support cursive writing where character boundary information is not available. Systems that are capable of generating both in and out of dictionary recognition hypotheses simultaneously, typically exhibit lower in-dictionary accuracy.
Thus, there is a need for handwriting input user interface that includes the advantages of providing a dictionary for normal written word recognition, while at the same time allows users to enter any out-of-vocabulary character string.